The present invention pertains to microlithography (imaging of a pattern, defined by a reticle or mask, onto a sensitive substrate). Microlithography is a key technique in the manufacture of microelectronic devices such as semiconductor integrated circuits, displays, and the like. More specifically, the invention pertains to microlithography performed using a charged particle beam (e.g., electron beam or ion beam) as an energy beam. Even more specifically, the invention pertains to devices and methods for correcting proximity effects in charged-particle-beam (CPB) microlithography.
The most common conventional microlithography technology used for fabricating integrated circuits is the so-called xe2x80x9coptical stepperxe2x80x9d employing ultraviolet light as an energy beam. Microlithography technology employing a charged particle beam is still limited, in a practical (and hence commercial) sense, to CPB xe2x80x9cwritingxe2x80x9d systems mainly used for producing reticles used as pattern masters in optical steppers. However, in view of the resolution limits of optical microlithography, CPB microlithography has received considerable attention as a possible successor technology to optical microlithography for reasons similar to the argument that electron microscopy achieves greater resolution than optical microscopy.
One reason for the delay in establishing CPB microlithography as a principal lithographic technology used for mass-production of patterned wafers is the low throughput currently obtainable with this technology. One group of techniques currently used for performing CPB microlithography includes the xe2x80x9cpartial-pattern block exposurexe2x80x9d techniques (e.g., cell projection, character projection, and block exposure). Partial-pattern block exposure is used mainly for transferring a pattern including an array containing a large number (typically thousands) of repeated individual pattern units, such as the memory cells on a memory chip. The repeated units normally are very small, typically about 5 xcexcm square on the substrate. To form the array, the repeated units are exposed over and over again within a region on the wafer corresponding to the chip. As readily understood, considerable time is required to expose the array in each chip, which results in low throughput. Also, this technique is not used to transfer non-repeated portions of the chip pattern. Instead, the non-repeated portions usually are exposed by direct writing using a variable-shaped beam. This need to exploit multiple different techniques to expose each chip further compromises throughput. As a result, the partial-pattern block exposure techniques currently do not provide the throughput required for mass-production wafer fabrication.
A technique offering prospects of substantially greater throughput than the partial-pattern block exposure techniques involves exposing, in a single xe2x80x9cshot,xe2x80x9d a reticle defining the entire pattern to be transferred to a chip or defining a pattern for multiple chips. The reticle is exposed with xe2x80x9cdemagnification,xe2x80x9d by which is meant that the reticle image as formed on the wafer is smaller (usually by an integer factor termed a xe2x80x9cdemagnification ratioxe2x80x9d) than the corresponding pattern as defined on the reticle. Whereas the throughput potentially achievable using this technique is at least as good as currently achievable using optical microlithography, this technique unfortunately has several serious problems. One problem is the current impossibility of fabricating a reticle configured to be exposed in a single shot of a charged particle beam. Another problem is the current impossibility of adequately correcting off-axis aberrations, especially in peripheral regions of the large image produced by the charged particle beam.
A more recently considered approach is termed xe2x80x9cdivided-reticlexe2x80x9d pattern transfer, which involves dividing the pattern, as defined on the reticle, into multiple individual exposure units usually termed xe2x80x9csubfields.xe2x80x9d Each subfield is exposed individually onto a respective region on the wafer. The subfield images are transferred to the wafer so that, after exposing all the subfields, the subfield images are xe2x80x9cstitchedxe2x80x9d together in a contiguous manner to form the entire chip pattern. As each subfield is exposed, corrections are made to achieve proper focus and reduction of aberrations (e.g., distortion) for the particular subfield. Divided-reticle pattern transfer allows exposures to be made over an optically wide field with much better resolution and accuracy than could be obtained by exposing the entire reticle in one shot. Although divided-reticle pattern transfer does not yet achieve the same throughput as optical microlithography, the throughput nevertheless is much better than obtainable using the partial-pattern block exposure technique.
Certain aspects of divided-reticle pattern transfer are shown in FIGS. 23 and 24. FIG. 23 depicts a wafer on which multiple chips have been exposed. As exposed, each chip comprises multiple xe2x80x9cstripes,xe2x80x9d and each stripe comprises multiple subfields arranged in rows. This same divided arrangement of stripes and subfields is used to define the pattern on the reticle. FIG. 24 depicts an actual exposure. For exposure, the reticle and wafer are mounted on respective stages (not shown but well understood in the art) configured to move the reticle and wafer horizontally (in the figure) as required for exposure. During exposure of a stripe (a portion of which is shown), the reticle stage and wafer stage both move along the longitudinal center line of the respective stripes. Movements of the reticle and wafer are at constant respective velocities (but in opposite directions) in accordance with the demagnification ratio. Meanwhile, the charged particle beam incident on the reticle (the beam upstream of the reticle is termed the xe2x80x9cillumination beamxe2x80x9d and passes through an xe2x80x9cillumination-optical systemxe2x80x9d to the reticle) illuminates the subfields on the reticle row-by-row and subfield-by-subfield within each row (the rows extend perpendicularly to the movement directions of the reticle and wafer). As each subfield is illuminated in this manner, the portion of the illumination beam passing through the respective subfield (now termed the xe2x80x9cpatterned beamxe2x80x9d or xe2x80x9cimaging beamxe2x80x9d) passes through a projection-optical system to the wafer.
During exposure of a stripe, to expose the rows and subfields within each row of the stripe in a sequential manner, the illumination beam is deflected at right angles to the movement direction of the reticle stage and the patterned beam is deflected at right angles to the movement direction of the wafer stage. After completing exposure of each row, the illumination beam is deflected in the opposite direction, as shown in FIG. 24, to expose the subfields in the next row of the stripe. This exposure technique reduces extraneous deflections of the beam and improves throughput.
Whenever a xe2x80x9csensitive substratexe2x80x9d (e.g., resist-coated wafer) is irradiated with a charged particle beam, backscattering of charged particles from the resist and substrate causes the actual exposure dose to vary according to the distribution of pattern elements in the proximity of the beam. This phenomenon commonly is known as a xe2x80x9cproximity effect.xe2x80x9d The proximity effect also arises from forward-scattering of incident charged particles into the resist. Forward-scattering and backscattering results in a net outward propagation (spreading propagation) of charged particles from the respective points of incidence through the resist. This spreading out of charged particles from the respective points of incidence applies exposure energy to areas of the resist (adjacent to points of incidence) where exposure is not desired. Current methods for solving this problem include adjusting the radiation dose to obtain the desired amount of accumulated energy on the substrate, as described in Japan Kxc3x4kai Patent Document No. Hei 11-31658, and modifying the profiles of individual pattern elements as defined on the reticle (xe2x80x9clocal resizingxe2x80x9d) so as to achieve desired xe2x80x9cas-projectedxe2x80x9d profiles of the elements on the substrate, as described in Jpn. J. Appl. Phys. 36:7477-7481, 1997.
Another problem is beam xe2x80x9cblurxe2x80x9d introduced by the CPB optical system of the microlithography apparatus. Blur is caused by geometric aberrations (e.g., spherical aberrations) similar to those encountered in conventional light-optical systems, and by Coulomb interactions (mutual repulsion) between individual particles in the propagating beam.
Geometric aberrations worsen with increased lateral distance from the optical axis. (The optical axis is the axis of the lens column of the illumination-optical system and projection-optical system.) In divided-reticle projection-transfer systems, in which the reticle subfields are exposed individually, blur increases with the increased lateral distance between the optical axis and the center of the subfield or other individual exposure unit being exposed. Within individual subfields, blur generally tends to be greater at the edges of the subfield than at the center of the subfield.
Blur due to Coulomb interactions tends to become more pronounced with increases in the surface area of individual exposed pattern elements, because larger pattern elements receive more beam current than smaller pattern elements. This blur results in unwanted variations in pattern linewidth as projected onto the substrate. More specifically, blur reduces the number of charged particles entering the intended incidence locus on the surface of the sensitive substrate. The effect of blur is similar to a proximity effect; namely, for example, the pattern-element lines are wider or narrower than intended.
Conventionally, measures for reducing blur are limited to applying deflection-magnitude corrections (i.e., correcting the lateral distance between the optical axis and the center of the exposed subfield) and correcting focus according to the net area of pattern elements, relative to total subfield area, in the particular subfield being exposed. Because the entire subfield is exposed simultaneously in the respective shot, in a subfield in which blur exhibits a large variation between the center and peripheral edges of the subfield, these conventional remedies cannot simultaneously correct for variations in blur. Consequently, due to uncorrected blur, some of the subfields inevitably have unwanted variations in the sizes of their respective pattern elements as projected onto the substrate.
Conventionally, whenever some of the projected subfields have a large degree of blur and others have only a small degree of blur, blur is increased intentionally in the latter subfields using a focus-correction lens. As a result, all the subfields as projected have roughly the same degree of blur and the same scatter coefficient based on proximity effects over the entire chip. The blur is corrected by altering the respective profiles of individual pattern elements on the reticle (xe2x80x9clocal resizingxe2x80x9d).
The conventional methods for reducing blur and proximity effects as summarized above achieve compensation of different degrees of blur and proximity effects between individual subfields. However, whenever the degree of blur at the center of a particular subfield is different from the degree of blur at the edge of the same subfield, because the edge and center are both exposed at the same instant, the conventional method summarized above is ineffective for correcting this difference. As a result, in some subfields, unintended variations in the sizes of individual pattern elements occur due to blur.
Also, in the conventional situation summarized above, even if the microlithography apparatus that is used produces some exposed subfields having low degrees of blur, any possibility of using those low-blur subfields to produce fine patterns on the substrate is elusive because of the blur intentionally imparted during exposure.
In view of the shortcomings of conventional apparatus and methods as summarized above, an object of the invention is to provide proximity-effect correction methods allowing projection of patterns with higher dimensional accuracy than obtainable conventionally.
According to a first aspect of the invention, methods are provided (in the context of a charged-particle-beam CPB microlithography method) for correcting proximity effects resulting from backscattering of charged particles in the resist-coated substrate. In one embodiment, the proximity effects are corrected by correcting pattern-exposure data including data on radiation dose and respective profiles and dimensions of pattern elements, so as to adjust respective positions of edges of the pattern elements as defined on the reticle. In this embodiment, data concerning blur of the charged particle beam irradiating the substrate is determined for selected subfields. The blur for a selected subfield is a function of beam deflection to expose the subfield and of location of one or more pattern elements within the subfield. From the determined blur, preliminarily corrected pattern-element exposure data are calculated. At one or more edges of a pattern element as exposed onto the substrate, exposure dose, as affected by backscattering, of the pattern element (as defined by the preliminarily corrected pattern-exposure data) is calculated. From the determined blur and calculated exposure dose, a distribution of exposure dose in the vicinity of pattern-element edges as projected onto the substrate is calculated. From the calculated distribution of exposure dose, a dose threshold value is determined. From the determined dose threshold value and the calculated distribution of exposure energy, positions of pattern-element edges (that actually will be formed on the substrate) are predicted. From the predicted positions of pattern-element edges, the pattern-element edge positions as defined on the reticle are adjusted to cause the pattern-element edges as projected to be situated in respective prescribed locations on the substrate.
Calculation of exposure dose as affected by backscatter can be performed in a manner in which, in a pattern region having a dimension substantially similar to a diameter of backscatter from an edge of the pattern element, a grid is defined for dividing the pattern region into sub-regions containing respective portions of the pattern element. For example, the sub-regions have dimensions that are from {fraction (1/100)} to ⅓ the backscatter diameter. Within a sub-region, the respective pattern-element portion is defined by a corresponding representative figure. The exposure dose, as affected by backscattering, is calculated based on the representative figure.
Determining blur can include determining a difference of blur from another location on the pattern from maximal blur for the pattern. In this instance, the correction is made, from doses contributed by proximity effects and slopes of edges of projected pattern elements, such that, at a prescribed dose threshold, the edges of the pattern elements are projected at their respective prescribed locations on the substrate.
According to another embodiment of a method for correcting proximity effects at time of reticle fabrication, beam blur (of the charged particle beam irradiating the substrate) is determined as a function of beam deflection and respective locations of pattern elements within an area of the pattern exposed in a respective shot. At one or more edges of a pattern element as exposed onto the substrate, exposure dose (as affected by backscattering) of the pattern element is determined. From the determined blur and the calculated exposure dose, a distribution of exposure dose in the vicinity of pattern-element edges as projected onto the substrate is calculated. From the calculated distribution of exposure dose, a dose threshold value is determined. From the determined dose threshold value and the calculated exposure dose, positions of pattern-element edges that actually will be formed on the substrate are predicted. From the predicted positions of pattern-element edges, the pattern-element edge locations to be defined on the reticle are adjusted to cause the pattern-element edges as projected to be situated in respective prescribed locations on the substrate.
In yet another embodiment of a method, according to the invention, for correcting a proximity effect, a design pattern is defined that is to be formed on the resist-coated substrate. A primary reticle pattern is obtained by enlarging the design pattern by the reciprocal of a demagnification ratio of the projection-exposure system. An energy profile DW(x) of the charged particle beam is calculated that would exist at the resist-coated substrate after passage through the primary reticle pattern and through the projection-optical system. The calculation of DW(x) takes into account a reduction of energy at the substrate due to blur of the charged particle beam during passage through the projection-optical system. A profile E(x) of cumulative exposure energy, due to backscatter occurring at the resist-coasted substrate being irradiated by a charged particle beam having the energy profile DW(x), is calculated. For an energy profile that is a sum of DW(x) and E(x), a development-energy threshold is set for the pattern elements as projected from the primary reticle pattern. Widths of pattern elements, that would be formed on the resist-coated substrate exposed with the charged particle beam from the primary reticle pattern, are calculated. From the calculated widths of pattern elements, the primary reticle pattern is corrected to form a for-projection pattern that will produce linewidths according to the design pattern. To such end, corrections are made to individual pattern elements as defined on the reticle, taking into account the beam blur versus location of edges of the respective pattern elements within an area to be exposed in a single shot.
According to another aspect of the invention, methods are provided (in the context of CPB microlithography) for producing a reticle defining pattern elements configured so as to correct proximity effects in the pattern as projected onto the resist-coated surface of the substrate. In an embodiment of such a method, data are produced for a primary reticle pattern as defined on a segmented reticle comprising subfields each defining a respective portion of the reticle pattern to be exposed onto the substrate in a respective shot. For each subfield, and according to the primary-reticle-pattern data, a distribution of blur at the substrate is determined that would exist if the pattern, as defined by the primary reticle pattern, were projected onto the substrate. From the determined distribution of blur from the primary reticle pattern, a distribution of beam energy that would exist at the substrate for each projected subfield is determined. From the determined distribution of beam energy as projected on the substrate for each subfield, a distribution of exposure energy in the resist-coated substrate for each subfield is determined, taking into account proximity effects and backscattering of the beam in the resist-coated substrate. From the determined distribution of exposure energy, a dose threshold for each subfield is determined. From the determined distribution of exposure energy and dose threshold, data defining locations of edges of pattern elements in the respective pattern portion defined by each subfield are generated. Hence, when the respective pattern portions are projected, the pattern elements will form at desired respective locations with desired profiles on the resist-coated substrate. Finally, from the generated data, the reticle is fabricated.
In another embodiment of a method for producing a reticle, data are produced on a primary reticle pattern as divided into subfields each defining a respective portion of the pattern to be projected onto a the substrate in respective shots. A distribution of beam blur is determined. The distribution is a function of beam deflection for irradiating a given subfield of the primary reticle pattern and of a distribution of pattern elements within the given subfield. Data regarding the determined distribution of beam blur are stored. A beam-energy profile at the substrate exposed with the primary reticle pattern is determined. The data regarding the distribution of beam blur are recalled, and a cumulative exposure-energy distribution at the substrate exposed with the primary reticle pattern is determined, taking into account proximity effects in the exposed primary reticle pattern and the data on the distribution of beam blur. From the determined distribution of cumulative exposure energy, a dose threshold is determined. At the dose threshold, edge locations of pattern elements of the primary reticle pattern as projected are calculated. Based on the calculated edge locations, the primary pattern data are corrected so that edge locations correspond to respective edge locations according to design specifications. The corrections are made by local resizing of pattern elements, by local dose corrections, or both. Based on the corrected pattern data, the reticle is fabricated.
In the foregoing method, the distribution of beam blur can be determined as a mathematical function or a data table that is stored. The distribution of blur can be calculated as an isotropic distribution or as an anisotropic distribution. The calculation of edge locations of pattern elements as projected can include taking into consideration the distribution of beam blur. Also, the correction to the primary reticle pattern data can include taking into consideration a dose level and slope of the corresponding beam-energy distribution at the substrate. In correcting the primary reticle data by taking into consideration the dose level and slope of the corresponding beam-energy distribution at the substrate, a selected pattern portion (of the primary pattern) can be obtained by dividing a selected pattern portion into subregions each containing a respective portion of a pattern element. The pattern-element portion in each subregion can be represented as a respective representative figure having an area equal to an area of the respective pattern-element portion. With respect to each representative figure, fogging due to backscattering is calculated. From the calculated fogging for individual subregions, fogging for the element in the selected pattern portion is calculated. Based on data regarding fogging for the pattern element, a distribution of exposure energy at the edges of the pattern element can be calculated. In addition, slopes of the exposure-energy distribution at the edges of the pattern element can be calculated, wherein the primary pattern data can be corrected to a degree determined from the calculated slopes of exposure-energy distribution.
According to another aspect of the invention, reticles are provided that are produced by any of the various methods of the invention.
According to yet another aspect of the invention, methods are provided for performing CPB microlithography. In these methods, a reticle such as any summarized above are provided.
In yet another embodiment of a method, according to the invention, for correcting proximity effects by local resizing of pattern elements defined on the reticle, beam blur is determined. Beam blur is determined for each subfield to be projected onto the substrate in a respective shot of the charged particle beam, and is determined at each of multiple locations in the subfield. The blur data is stored in a retrievable form. The blur data are retrieved and utilized in calculations to modify profiles of respective pattern elements defined in the subfields as required to correct proximity effects that otherwise would be produced by projection of the subfields onto the substrate. Using the results of these calculations, the profiles of the respective pattern elements are modified.
Again, as noted above, the blur data can be stored as a data table or as a mathematical function, for example. Typically, the blur data differ from subfield to subfield of the pattern.
According to another aspect of the invention, computer-readable media are provided that comprise a computer program for executing any of the methods according to the invention.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.